1. Field of the Invention
This invention relates to transparent sheets formed from blends of polycarbonate and polyethylene terephthalate resins, a process for their production and containers formed therefrom.
2. Discussion of the Prior Art
Polyethylene terephthalate (sometimes referred to as "PET") resins can be employed to prepare transparent film and sheet. Usually the resin is extruded into an amorphous flat sheet, which is then biaxially stretched and thereafter heat set to impart a desired degree of crystallization to the sheet. Such biaxially oriented and crystallized products are strong and clear but cannot readily be formed into containers since the process of biaxially stretching removes most of the extensibility of the sheet. If amorphous PET sheet is produced by rapid cooling of the molten sheet, a clear and transparent product may be obtained which is formable into containers. These containers soften at too low a temperature to permit their use in hot-filled food packaging applications where the filling may typically be at a temperature of about 150.degree. to 180.degree. F. or greater which facilitates rapid filling of viscous products as well as destroying bacteria. However, for lower temperature filling food packaging, at temperatures from about 100.degree. to 140.degree. F., these containers are useful.
If PET sheet is produced by slow cooling of the molten resin, the product obtained is partially crystallized, milky and brittle and hence unsuited for container fabrication. If PET sheet is rapidly cooled, with a cooled surface, such as a cooling roll, deposits of decomposition products such as terephthalic acid rapidly form on the cooling surface.
The effect of moisture on polyethylene terephthalate in the melt state is well known, in that rapid hydrolytic cleavage of the polymer molecules takes place, the number of such cleavages being in direct proportion to the quantity of water present. This, of course, results in a drop of the average molecular weight of the polymer. In some commercial extrusion processes, some moisture is always present during extrusion at a level generally of 0.005-0.02 percent due to the inefficiencies of commercial hopper driers and the extreme rapidity with which PET resin absorbs moisture. Very costly vacuum drying equipment and elaborate nitrogen blanketing is required to reduce the moisture level to zero. However, the drop in molecular weight experienced does not affect the usefulness of the sheet product.
The critical effect of moisture is that some hydrolysis occurs on the end unit of the polymer resulting in the liberation of terephthalic acid. In order to produce clear amorphous sheet, after leaving the die the melt is rapidly quenched by passing it through a three-roll stack whose surface temperature is kept in the range of 50.degree.-175.degree. F. These temperatures are sufficiently low to cause condensation of the terephthalic acid on the roll surfaces. This deposit becomes increasingly difficult to remove as the run proceeds as the hot sheet "bakes" it to the rolls, particularly the center roll. Before the invention it was necessary to continuously wipe the rolls by hand and, because of the design of the equipment, the center roll was difficult and dangerous to wipe unless the line was stopped, resulting in loss of production. The effect of this deposit is to cause poor polishing of the sheet, giving rise to haze and streaks and thus producing an unsightly product.
Although it is known that lower intrinsic viscosity PET resins may be modified by the addition of polycarbonates, as is described in U.S. Pat. No. 3,218,372 to Okamura et al., issued in 1965, in order to increase the hardness, strength and electrical properties of the molding material, that disclosure was limited to much larger amounts of polycarbonate resin, i.e., a minimum of 10 percent.
In copending U.S. Ser. No. 625,299, filed Oct. 23, 1975 now abandoned, a sheet intended for hot filled applications, 150.degree. to 180.degree. F. or higher, uniformly blended from about 80 to 95 percent PET and 20 to 5 percent polycarbonate is disclosed.
U.S. Pat. Nos. 3,956,229 and 3,975,485, hereby incorporated by reference, in toto, there is described film and sheet formed from blends of 60 to 85 parts of PET having an intrinsic viscosity of at least about 0.90 and 40 to 15 parts of a polycarbonate resin. Such film or sheet, which has a degree of crystallinity in the range of about 20 to 40 percent, is essentially nonoriented and may be thermoformed into cook-in trays and like articles. The film or sheet disclosed therein is formed by blending the polymers, extruding the blend at a temperature above about 500.degree. F. onto a moving support and cooling the support to a surface temperature of about 225.degree. to 380.degree. F. Although such film and sheet have requisite strength and toughness to be utilized for cook-in tray applications, such sheets have a very high degree of haziness and consequently would not be suitable for applications wherein a clear sheet is desired. In U.S. Pat. No. 3,975,355, there is described film or sheet which comprises a similar blend of PET and polycarbonate and about 5 to 20 parts by weight of a nonacidic silica filler such as novaculite. However, such film or sheet likewise has a degree of haziness which precludes its utilization is hot filled or even for fillings heated to 100.degree. to 140.degree. F., wherein a clear and transparent sheet is required.
In U.S. Pat. No. 3,720,732, a disclosure limited to a biaxially oriented PET film discusses adding small amounts of polycarbonate to the resin to improve heat stability and slip properties. The invention of this application is directed to unoriented, amorphous sheet and improved processing thereof, rather than product properties of oriented film.